Pictures taken with flash often show a well exposed foreground subject against a black background. Using slow sync flash minimizes this problem by using flash to illuminate the foreground subject and then leaving the shutter open longer than usual to lighten the background. Many cameras have a night portrait or a night landscape mode that uses this effect. On other cameras you use it by selecting shutter-priority mode and then selecting a slow shutter speed such as 1/20, but experiment because the results are hard to predict.

Slow synchro flash created this image that's both sharp and blurred at the same time.

In many cases, the slow shutter speed used in this mode allows blur from rapidly moving objects or camera shake to appear as blur in the images. To avoid blur, use a tripod and photograph static subjects. Or, use this effect creatively. A short flash burst combined with a long shutter speed gives interesting effects. The flash freezes objects sharply, and then in the dim light, camera or subject movements blur the image to create streaks.

When using slow sync flash, some cameras let you take advantage of front or rear curtain sync (also called first and second curtain sync). As discussed on page 101 your choice determines whether the flash fires just after the front curtain opens or just before the last curtain closes. The differences can be striking. For example, when photographing a moving car at night, front curtain sync will cause the taillights to have a forward streak and rear curtain sync will cause them to have a receding streak.

The flash was set to slow sync so the bride in the foreground is frozen while the lights in the background are blurred and streaked.